Avalanche photodiode

ABSTRACT

An avalanche photodiode includes at least one crystal layer having a larger band-gap than that of an absorption layer formed by a composition or material different from that of the absorption layer formed on a junction interface between a compound semiconductor absorbing an optical signal and an Si multiplication layer, and the crystal layer may be intentionally doped with n or p type impurities to cancel electrical influences of the impurities containing oxides present on the junction interface of compound semiconductor and surface of Si.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an avalanche photodiode and moreparticularly to a fast, highly sensitive, wideband avalanche photodiodewith a large gain for use in optical communication.

2. Description of the Related Art

The avalanche photodiode is a light receiving device with a built-infunction of amplifying an optical signal and, because of its highsensitivity and fast operation, has found a wide range of applicationsas an optical communication light receiving device. The amplificationfunction of the avalanche photodiode is realized by taking advantage ofan avalanche breakdown phenomenon that occurs in semiconductors. Aprinciple by which an amplification occurs during the avalanchebreakdown is briefly explained as follows.

Electrons or holes moving in a semiconductor are scattered by a crystallattice when they strike it. Applying a large electric field to thesemiconductor accelerates these carriers, resulting in an increase intheir moving speed. As the moving speed of the carriers in thesemiconductor increases and their kinetic energy is higher than abandgap of the semiconductor, a probability of breaking bonds of latticeincreases when they hit the crystal lattice, newly creating free-movingelectron-hole pairs. An atom with its bonds broken loses electriccharges and looks as if it is ionized. This phenomenon is thereforecalled an impact electrolytic dissociation or impact ionization, and ameasure of how many electron-hole pairs are generated by the impactionization after an electron or hole has traveled a unit distance isalso called an ionization rate. A ratio of an ionization rate based onelectrons to an ionization rate based on holes is further called anionization rate ratio.

Newly created carriers (electrons or holes) produced by the impactionization are also accelerated by the electric field and acquire akinetic energy, with subsequent impact ionizations further creating newcarriers. As the impact ionization repetitively occurs, the number ofcarriers increases rapidly, creating a large current. This is thephenomenon called an avalanche breakdown. In a semiconductor that isapplied an electric field of a magnitude just below the avalanchebreakdown, an injection of carriers, even in a small number, can producea large number of new carriers through the impact ionizations, resultingin a sudden increase in current. That is, a large current can beobtained even with an injection of a small number of carriers. This is aprinciple by which amplification is accomplished during the avalanchebreakdown. The avalanche photodiode uses photo-induced carriers producedby an optical absorption for the carrier injection that triggers thisphenomenon.

As well known, an important factor in terms of a high-speed response ofthe avalanche photodiode is the ionization rate ratio. The more theionization rate ratio is away from unity, the better the performance ofthe avalanche photodiode becomes. Conversely, as the ionization rateratio approaches unity, the amplification rate at high speeddeteriorates, making it impossible to produce an avalanche photodiodewith a good performance. Since infrared light is used in a high-speedoptical communication, the fabrication of the light receiving device hasso far used compound semiconductors, such as InP and InGaAs. However,the ionization rate ratio of InP, a typical compound semiconductor usedin optical communication, is 0.5, relatively close to unity. Even withInAlAs the ionization rate ratio is 4 or 5 at most. Thus, an applicablefrequency is about 10 GHz at most. As a result, a satisfactoryperformance cannot be obtained for high-speed devices of 40 GHz orhigher.

On the other hand, Si has a very large ionization rate ratio rangingfrom 10 to more than 100 and thus can make a fast, highly sensitiveavalanche photodiode. However, since Si cannot absorb light in aninfrared frequency range used for optical communication, Si has not beenable to be used for optical communication.

To overcome this drawback of the Si avalanche photodiode, an attempt hasbeen made to combine Si with a compound semiconductor that has asensitivity in the infrared range. For example, epitaxially growing acompound semiconductor on Si has been explored for a couple of decadesnow. However, no crystal with a satisfactory quality has been realizedfor practical use.

An example method for alleviating this quality problem of such acompound semiconductor on Si is disclosed in U.S. Pat. No. 6,384,462B1,which is briefly explained with reference to FIG. 2. In this patent, anavalanche photodiode is formed by directly fusing a Si multiplicationlayer 23 onto an InGaAs layer 22 epitaxially grown on a compoundsemiconductor substrate 21, as shown in FIG. 2. Further, by using ionimplantation and diffusion techniques, a contact layer 24 and a guardring 25 are formed. The use of the fusing technique keeps thecrystalline structure of both the compound semiconductor and Si intact,so a high quality light receiving device can be obtained.

In the structure described above, however, the Si multiplication layeris directly fused at elevated temperatures to the InGaAs layer of a lowcarrier concentration that absorbs optical signal. Normally, on aninterface of a junction between the InGaAs layer and the Simultiplication layer, there are many impurities including oxides. Theseimpurities infiltrate into the InGaAs layer near the junction during thefusing process. As a result, the carrier concentration in the InGaAslayer near the junction increases, resulting in a high electric fieldbeing applied. The InGaAs layer has a narrow bandgap, so that when it isapplied a high electric field, a dark current increases, degrading thesensitivity down to a level not suitable for practical use. In fact, ina device which has a Si multiplication layer directly fused to an InGaAslayer, the dark current exceeds a microampere, making the sensitivity ofthe device three or more orders of magnitude worse than those ofconventional avalanche photodiodes in practical use. Further, a highelectric field gives rise to a problem of causing an avalanche breakdowneven in the InGaAs layer and thus degrading a high-speed response.

An object of the present invention is to provide an avalanche photodiodehaving a low dark current, a high sensitivity and a high speed and madeof a combination of a compound semiconductor and Si, and to provide amethod of manufacturing the same.

SUMMARY OF THE INVENTION

The avalanche photodiode of this invention has a structure in which, inan interface between a compound semiconductor that absorbs an opticalsignal (referred to as an absorption layer) and a Si multiplicationlayer, at least one crystal layer formed of a composition or materialdifferent from that of the absorption layer and having a larger bandgapthan that of the absorption layer (referred to as an interface layer) isformed.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor light receivingdevice as a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a semiconductor light receivingdevice as a conventional example.

FIG. 3 is a cross-sectional view of a semiconductor light receivingdevice as a second embodiment of the present invention.

FIG. 4 is a cross-sectional view of a semiconductor light receivingdevice as a third embodiment of the present invention.

FIGS. 5A to 5I are explanatory diagrams showing a process of fabricatinga semiconductor light receiving device of the third embodiment of thepresent invention.

FIG. 6 is a graph showing a wavelength dependency of an absorptioncoefficient of InGaAs.

FIG. 7 is a cross-sectional view of a semiconductor light receivingdevice as a fourth embodiment of the present invention.

FIG. 8 is a cross-sectional view of a semiconductor light receivingdevice as a fifth embodiment of the present invention.

FIG. 9 is a cross-sectional view of a semiconductor light receivingdevice as a sixth embodiment of the present invention.

FIG. 10 is a cross-sectional view of a semiconductor light receivingdevice as a seventh embodiment of the present invention.

FIG. 11 is a cross-sectional view of a semiconductor light receivingdevice as a eighth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1

FIG. 1 illustrates an example structure of the avalanche photodiode ofthis invention. Denoted 11 is a Si substrate (n-type, 2xE18 cm⁻³), 12 aSi multiplication layer (n-type, 1E15 cm⁻³, 0.2 μm), 13 an InAlAsinterface layer (p-type, 1E18 cm⁻³, 0.05 μm), 14 an InGaAs absorptionlayer (p-type, 2E15 cm⁻³, 1.2 μm), 15 an InAlAs capping layer (p-type,2E18 cm⁻³, 1 μm), and 16 an InGaAs contact layer (p-type, 5E19 cm⁻³, 0.1μm). Reference number 17 represents a SiN film protecting the surface ofthe device. Reference number 18 is a metal electrode. The structure ofthe device shown is of a surface illuminated type, and an optical signalenters from a surface of the Si substrate 11 or the contact layer 16.The light receiving surface may be provided with a non-reflective coatfilm or an appropriate window structure or lens to enhance the opticalsignal receiving efficiency.

Embodiment 2

FIG. 3 illustrates another embodiment of this invention. In thisembodiment, the device has a planar structure for improved reliability.Denoted 31 is a guard ring and a p-type impurity is doped through ionimplantation or diffusion.

Embodiment 3

FIG. 4 illustrates still another embodiment of this invention. In thisembodiment the basic structure of the element is similar to that of FIG.1 except that a guard ring is provided for improved reliability.Designated 41 is an InGaAlAs interface layer (p-type, 1E18 cm⁻³, 0.05μm). The composition of InGaAlAs is adjusted so that the interface layerhas a bandgap wavelength of 1.1 μm (equivalent to 1.13 eV) to prevent anoptical signal of a 1.3-μm band from being absorbed. Denoted 42 is aguard ring formed of high-resistance InP. The guard ring may be p- orn-type InP if the carrier concentration is low.

A process of manufacturing this structure will be explained by referringto FIGS. 5A to 5I. First, a compound semiconductor and Si to be joinedtogether are prepared separately. As shown in FIG. 5A, a highlyresistive Si multiplication layer 52 with a low carrier concentration isepitaxially grown on an n-type Si substrate 51 through an appropriatemethod. Alternatively, an n-type impurity may be diffused into a highlyresistive Si substrate to form the same structure. It is also possibleto diffuse a p-type impurity into an n-type Si substrate to increase theresistance of the surface and thereby form the same structure. Next, asshown in FIG. 5B, this structure is formed into a trapezoidal shape(mesa) as by photolithography and dry or wet etching. The dimensions ofthe mesa structure need to be set to produce a proper capacity for highfrequency use. In this embodiment, the mesa structure is shaped like atruncated cone which at its top measures about 25 μm in diameter for usein a 10 GHz range. Then, a dielectric film 53 of SiN or SiO₂ is formedover the surface by a proper chemical vapor deposition method to protectthe surface. In the case of SiO₂, the dielectric film may be formed by athermal oxidation method. Next, as shown in FIG. 5C, only the topportion of the dielectric film is removed by photolithography and dry orwet etching to expose the surface 54 of Si. Now, the preparation of Siis complete.

The compound semiconductor is prepared as follows. First, as shown inFIG. 5D, a p-type InGaAs contact layer 56 (with a carrier concentrationof 5E19 cm⁻³ and a thickness of 0.1 μm), a p-type InGaAlAs capping layer57 (2E18 cm⁻³, 1 μm), a p-type InGaAs absorption layer 58 (1E15 cm⁻³, 1μm) and a p-type InGaAlAs interface layer 59 (1E18 cm⁻³, 0.05 μm) areepitaxially grown in that order over the InP substrate 55 by a molecularbeam epitaxy. These layers are adjusted in their composition so as tohave a lattice match with the InP substrate, and are also doped with Be,a p-type impurity, to control their carrier concentrations.

The composition of InGaAlAs used in the cap and interface layers isadjusted so that its bandgap will be 1.1 μm. This adjustment is made toensure that the device does not absorb light of a 1.3-μm band, whichrepresents an optical signal. FIG. 6 shows a relation between an opticalabsorption coefficient and a wavelength of light for InGaAs. It can beseen from this graph that InGaAs absorbs almost no light when theoptical wavelength is about 0.1 μm longer than its bandgap wavelength.Thus, if the bandgap wavelength is set shorter than 1.2 μm, InGaAlAsused in the cap and interface layers no longer absorbs a 1.3-μm bandoptical signal, thus avoiding an unwanted loss of the optical signal.That is, the composition of InGaAlAs used in the cap and interfacelayers need only have a bandgap wavelength shorter than 1.2 μm, and itsbandgap wavelength is not limited to 1.1 μm.

However, InGaAlAs used in the cap and interface layers also has a limitvalue on a shorter wavelength side of the bandgap wavelength, which isrestricted by a difference in bandgap between it and the InGaAsabsorption layer. That is, when the difference in bandgap betweenInGaAlAs used in the cap and interface layers and the InGaAs absorptionlayer becomes too large, the electrons and holes cannot ride over theenergy difference at the interface and build up there, resulting in aloss of a high-speed response, a so-called pileup phenomenon. Thus, thebandgap of InGaAlAs used in the cap and interface layers must not be setexcessively large. Normally, to obtain a 10-GHz response speed, theenergy difference in a conduction or valence band between the bandgap ofInGaAlAs used in the cap and interface layers and the bandgap of theInGaAs absorption layer needs to be set to about 0.5 eV. Based on this,the limit value on the shorter wave-length side of the bandgapwavelength of InGaAlAs used in the cap and interface layers iscalculated to be approximately 700 nm.

These layers may be grown by a metalorganic vapor phase epitaxy or aproper chemical vapor deposition. The p-type dopant may be Zn. Next,this structure is processed by photolithography and dry or wet etchinginto a trapezoidal shape (mesa), as shown in FIG. 5E. A top of thetruncated cone structure thus formed has a diameter of about 25 μm, asin FIG. 5B. Now, the preparation of the compound semiconductor iscomplete.

Next, Si 510 of FIG. 5C and the compound semiconductor 511 of FIG. 5E,prepared as described above, are joined as follows. As shown in FIG. 5F,Si of FIG. 5C and the compound semiconductor of FIG. 5E are arranged sothat their top portions oppose each other and, in this condition, theyare placed in a radio frequency plasma system. A small amount of argongas is introduced into a chamber of the system to clean the surfaces ofthe structures to be joined. Immediately after cleaning, the topportions are brought into contact to join Si of FIG. 5C and the compoundsemiconductor of FIG. 5E. This joining may be done by heating though itcan be performed at an ordinary temperature. Then, the joined structureis immersed in a weak hydrochloric acid-based etching liquid toselectively remove unwanted InP substrate. Then, as shown in FIG. 5G,the combined structure is subjected to the photolithography and dry orwet etching to process only the compound semiconductor into atrapezoidal shape again. After this, as shown in FIG. 5H, a dielectricmask 512 is formed by photolithography and dry or wet etching. This isfollowed by a highly resistive InP layer 513 being grown by ametalorganic vapor phase epitaxy or a proper chemical vapor deposition.Then, as shown in FIG. 5I, the dielectric mask is removed, after which aSiN film 514 for the protection of the entire device is formed by theplasma chemical vapor deposition and a hole for electrode connection isformed in the SiN film by photolithography. Then, a metal electrode 515is formed by vapor deposition, photolithography and liftoff process. Inthe last step, a non-reflective coat 516 is formed over the Si substratesurface which constitutes a light incident surface. Now, the lightreceiving device is complete.

When a reverse bias was applied to the device fabricated in this manner,a breakdown voltage Vb was 35 V and a dark current at 32 V, about 90% ofthe breakdown voltage, was as low as 50 nA. As for the high frequencycharacteristic, a multiplication factor of 10-GHz optical signal was 25at maximum and uniform within a light receiving range. Further, in areverse bias conduction test at an elevated temperature (200° C., 100 mAconstant), a voltage variation after 1000 hours was less than 1 V, and abreakdown voltage and a dark current at room temperature showed nochange from those before the test.

Embodiment 4

FIG. 7 shows yet another embodiment of this invention. The device ofthis embodiment has a structure similar in cross section to that ofEmbodiment 1 of FIG. 1, except that it is shaped like a waveguide. InFIG. 7, parts identical with those of FIG. 1 are given like referencenumbers. While in Embodiment 1 an optical signal strikes the substrateat right angles or at angles close to 90 degrees to it, this embodimenthas the optical signal enter the substrate parallel or nearly parallelto it. This device has a high speed and sensitivity of 40 GHz or higherand is suited for surface mounting.

Embodiment 5

FIG. 8 shows a further embodiment of this invention. The device of thisembodiment has a surface illuminated type structure similar to that ofEmbodiment 1, except that a compound semiconductor substrate is used asa base on which a Si multiplication layer is formed. Denoted 81 is anInP substrate (n-type, 2xE18 cm⁻³), 82 an InGaAs optical absorptionlayer (n-type, 2E15 cm⁻³, 1.2 μm), 83 an InGaAsP interface layer(n-type, 1E18 cm⁻³, 0.05 μm), 84 a Si multiplication layer (p-type, 1E15cm⁻³, 0.2 μm), and 85 a Si contact layer (p-type, 2E18 cm⁻³, 0.1 μn).The composition of InGaAsP is adjusted for the same reason as Embodiment3 so that it has a bandgap wavelength of 1.1 μm to prevent an opticalsignal of a 1.3 μm band from being absorbed.

Embodiment 6

FIG. 9 shows a further embodiment of this invention. Instead of a simpleSi substrate, the device of this embodiment uses a substrate formed witha Si or SiGe integrated circuit and has an avalanche photodiode similarto Embodiment 4 formed on that substrate. Denoted 91 is a preamplifiermade of a Si or SiGe integrated circuit on a Si substrate, and 92 anavalanche photodiode of FIG. 7. It is noted that a single substrate iscommonly used as the Si substrate 11 and the integrated circuit Sisubstrate 91.

Embodiment 7

FIG. 10 shows a further embodiment of this invention. This embodimentrepresents an example optical module having the avalanche photodiode 101of FIG. 4, a preamplifier integrated circuit device 102 and an opticalfiber 103 all accommodated in a single case 104.

Embodiment 8

FIG. 11 shows a further embodiment of this invention. This embodimentrepresents an example optical receiver having the optical module 110 ofFIG. 10 mounted on a package 111 incorporating an analog-digitalconverter and a decoder.

With the embodiments of this invention, even if an electric fieldstrength at an interface between Si and a compound semiconductor fusedtogether becomes abnormally high due to an effect of impurities presentat the interface, a large bandgap of the compound semiconductor materialat the interface can minimize an increase in a dark current. Bydeliberately doping impurities in the interface layer to nullifyelectric influences of the interface impurities, it is possible tosuppress electric field anomalies at the interface. As a result, ahighly sensitive, fast avalanche photodiode for optical communicationswith a much lower dark current can be realized.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An avalanche photodiode comprising: an absorption layer absorbinglight to create carriers; and a multiplication layer multiplying thecreated carriers, wherein the multiplication layer is formed of Si andthe absorption layer is formed of a compound semiconductor, and whereina semiconductor interface layer having a wider band-gap than that of theabsorption layer is formed between the multiplication layer and theabsorption layer.
 2. An avalanche photodiode according to claim 1,wherein the absorption layer is formed of an InGaAs mixed crystal orInGaAlAs mixed crystal or InGaAsP mixed crystal, and the semiconductorinterface layer is formed of the InGaAlAs mixed crystal or InGaAsP mixedcrystal.
 3. An avalanche photodiode according to claim 1, wherein theabsorption layer is formed of an InGaAs mixed crystal or InGaAlAs mixedcrystal or InGaAsP mixed crystal, and the semiconductor interface layeris formed of InP or GaAs.
 4. An avalanche photodiode according to claim1, wherein the absorption layer is formed of a semiconductor containingSb.
 5. An avalanche photodiode according to claim 1, wherein a junctionbetween the multiplication layer and the semiconductor interface layeris formed by a fusion.
 6. An optical module mounting an avalanchephotodiode, said avalanche photodiode comprises: an absorption layerabsorbing light to create carriers; and a multiplication layermultiplying the created carriers, wherein the multiplication layer isformed of Si and the absorption layer is formed of a compoundsemiconductor, and wherein a semiconductor interface layer having awider band-gap than that of the absorption layer is formed between themultiplication layer and the absorption layer.
 7. An optical receivermounting an avalanche photodiode, said avalanche photodiode comprises:an absorption layer absorbing light to create carriers; and amultiplication layer multiplying the created carriers, wherein themultiplication layer is formed of Si and the absorption layer is formedof a compound semiconductor, and wherein a semiconductor interface layerhaving a wider band-gap than that of the absorption layer is formedbetween the multiplication layer and the absorption layer.